Term long-run marginal cost Definition: The change in the long-run total cost of producing a good or service resulting from a change in the quantity of output produced. Like all marginals, long-run marginal cost is the increment in the corresponding total. What's most notable about long-run marginal cost, however, is that we are operating in the long run. Unlike the short run, in which at least one input is fixed, there are no fixed inputs in the long run. As such, there is only variable cost. This means that long-run marginal cost is the result of changes in the cost of all inputs.